Solar cell and method for producing the same

ABSTRACT

A solar cell  1  has a number of grooves  2  formed in parallel with each other on a first main surface  24   a  of a silicon single crystal substrate. An electrode  6  is formed on the inner side face of each groove  2  on one side. Each groove  2  is formed in the direction in disagreement with the &lt;110&gt; direction on the first main surface  24   a . This raises mechanical strength of the solar cell  1 . The direction of formation of the grooves  2  preferably crosses the &lt;110&gt;direction nearest to the direction of formation at an angle of 4°˜45° on the acute angle side.

TECHNICAL FIELD

[0001] This invention relates to a solar cell excellent in themechanical strength, and a method of fabricating the solar cell.

BACKGROUND ART

[0002] A method of fabricating a solar cell according to the OECO(Obliquely Evaporated Contact) process is disclosed for example inRenewable Energy, Vol. 14, p.83 (1998). The OECO process is a method offabricating solar cells proposed by R. Hezel et al. of Institut fürSolarenergieforschung GmbH Ha meln/Emmerthal (ISFH), Germany. Arepresentative structure of a light-receiving surface of the OECO solarcell is schematically shown in FIG. 2 (a solar cell fabricated by theOECO process may occasionally be referred to as OECO solar cell,hereinafter). The OECO solar cell is configured so that a plurality ofparallel grooves are formed on the main surface of a silicon singlecrystal substrate, which will serve as the light-receiving surface 3later, and so that electrodes 6 for extracting output are formed on theinner side faces of the individual grooves on a single side as viewedalong the width-wise direction of the grooves. This constitutionsuccessfully reduced the shadowing loss of the solar cell to as small asapprox. 5% of the total light-receiving area. Because a typical solarcell having the electrodes formed by the screen printing methodgenerally suffers from a shadowing loss of as large as approx. 12%, itis understood that the OECO solar cell has a sharply reduced shadowingloss, and that a large energy conversion efficiency is attainable.

[0003] In recent years, there is a strong demand for cost reduction infabrication of the solar cells. More specifically, thinning of solarcells can reduce the amount of single crystal silicon per unit area usedfor the solar cells, and can reduce the cost to some extent. Thinning ofthe OECO solar cell, which requires a large number of grooves to beformed on the main surface, however undesirably tends to reduce themechanical strength.

[0004] Besides those described in the above, there are known varioussolar cells having modified shapes of the electrodes formed on thelight-receiving surface or back surface of the cell in order to improvethe conversion efficiency. One publicly-known example of the solar cellis such as having grooves or bottomed holes for forming electrodecontact mechanically carved or bored in the semiconductor single crystalsubstrate, and having metal for composing the electrodes filled in thegrooves or bottomed holes. This type of cell was presented by tworesearch groups at the 28th IEEE Photovoltaic Specialists Conferenceheld in Anchorage in 2000.

[0005] The method by which the groove portions for forming electrodecontact of the solar cell are mechanically carved, and the method bywhich the bottomed holes for forming electrode contact are mechanicallybored were proposed independently by Institut für SolarenergieforschungGmbH Hameln/Emmerthal, Germany, and Fraunhofer Institute for SolarEnergy Systems ISE, Germany, respectively. Specific procedures forcarving the groove portions for forming electrode contact are such asfollows. First, a plurality of nearly-parallel groove portions forforming electrode contact are mechanically carved on a semiconductorsingle crystal substrate (e.g., silicon single crystal substrate) havingan insulating film such as a silicon oxide film (or silicon nitridefilm) formed thereon. Depth of the groove portions is set to 5 to 50 μm,and width thereof to several-hundred micrometers or around. The grooveportions can be carved by scanning once or several times over thesubstrate with a high-speed rotary blade having several hundreds tothousands cutting edges. After the carving of the groove portions, ametal is uniformly deposited on the main surface to thereby form anelectrode layer.

[0006] It is also possible to form bottomed holes for forming theelectrode contact so as to be linearly aligned at regular intervals.Depth of the bottomed holes herein is again set to 5 to 50 μm similarlyto the case where the groove portions are formed, and diameter of theopening of the bottomed holes is set to several-hundred micrometers oraround. This type of bottomed holes can be bored by irradiatingpredetermined sites with KrF excimer laser, Nd:YAG laser or the like.

[0007] Thus-fabricated solar cells, being passivated with the insulatingfilm in the non-contact area of the surface thereof, are advantageous insuppressing surface recombination of photo-generated carriers, and inconsequently raising the conversion efficiency of the solar cells. Thisprocess is also advantageous in that the groove portions and bottomedholes for forming electrode contact can be formed in a relatively simplemanner because formation thereof needs no photolithographic technique.

[0008] On the other hand, strong demands focused on the solar cells atpresent are improvement in the energy conversion efficiency and costreduction. Among others, the cost reduction can be realized by thinningof the solar cells to thereby reduce the amount of silicon singlecrystal substrate used for the cells. Thinning of the semiconductorsingle crystal substrate, however, undesirably lower the mechanicalstrength of the resultant solar cells. This inventors further revealedthat formation of the electrodes by carving or boring the grooveportions or bottomed holes in the semiconductor single crystal substrateinevitably causes damages to the substrate per se, and this may furtherdegrade the mechanical strength.

[0009] It is therefore a subject of this invention to provide a solarcell excellent in the mechanical strength, and a method of fabricatingthe solar cell.

DISCLOSURE OF THE INVENTION

[0010] As a solution to the aforementioned subject, a solar cellaccording to a first aspect of this invention is configured so as tohave a plurality of grooves nearly parallel with each other formed on afirst main surface of a semiconductor single crystal substrate having asurface orientation of nearly {100}, each of the grooves having anelectrode for extracting output disposed on the inner side face thereofon one side in the width-wise direction thereof (referred to as the OECOsolar cell, hereinafter), and the grooves being formed on the first mainsurface in directions in disagreement with the <110> direction.

[0011] In the conventional process of fabricating the aforementionedOECO solar cells, no attention has been paid on the direction offormation of the grooves formed on the main surface of the substrate.Investigations by this inventors, however, found out that when a largenumber of grooves are formed along the <110> direction on the majorsurface of a semiconductor single crystal substrate having a surfaceorientation of nearly {100} (simply referred to as the {100} substrate,hereinafter), the grooves may have portions where stress tends toconcentrate when viewed along its sectional contour, and the substratemay readily cleave along the grooves and result in fracture even underan action of slight external force when a lot of damage produced duringthe groove formation remains in the substrate.

[0012] Therefore in the first aspect of this invention, direction offormation of the grooves formed on the first main surface of the {100}substrate is set in disagreement with the <110> direction. Thissuccessfully raises the mechanical strength of the substrate andconsequently of the resultant solar cells to a large extent, andeffectively prevents or suppresses nonconformities such as factures fromoccurring during handling of the final products or intermediate productsof the solar cells even when the substrate has an extra-thin design.

[0013] Next, a solar cell according to a second aspect of this inventionis configured so as to have a plurality of filled electrode lines,having an electric conductor for composing electrodes for extractingoutput filled therein, on at least either main surface side of asemiconductor single crystal substrate having a surface orientation ofnearly {100}, and the filled electrode lines are formed in directions indisagreement with the <110> direction on the main surface in order tosolve the aforementioned subject.

[0014] Moreover, a method of fabricating a solar cell of this invention,intended for fabricating the solar cell according to the second aspect,comprises a step of forming a plurality of filled electrode lines,having an electric conductor for composing electrodes for extractingoutput filled therein, on at least either main surface side of asemiconductor single crystal substrate having a surface orientation ofnearly {100}, in directions in disagreement with the <110> direction onthe main surface. The electric conductor for composing electrodes inthis invention may comprise a metal layer, transparent conductive layer,or a stack of these layers.

[0015] It is to be noted that the “filled electrode line” in the contextof this invention is a general term for expressing the one fabricated byforming recessed portions on the main surface of the semiconductorsingle crystal substrate so as to concave the main surface, and byfilling the recessed portions with an electric conductor for composingthe electrodes, where the recessed portions are formed on the mainsurface of the semiconductor single crystal substrate so as to bealigned in a linear pattern. For example, the filled electrode line maybe such as comprising a plurality of grooves formed on the main surfaceof the semiconductor single crystal substrate, and an electrodeconductor for composing the electrodes filled in the grooves. Anotherpossible example relates to the one fabricated by forming bottomed holeson the main surface of the semiconductor single crystal substratelinearly aligned at regular intervals, and by filling the recessedportions with an electric conductor for composing the electrodes. It isto be noted now that the direction of formation of the filled electrodeline is defined as the linear direction along which the recessedportions are formed in a linear pattern. For example, for the case wherethe groove portions are formed as the recessed portions, the directionof formation is defined as the longitudinal direction of the grooveportions, and for the case where the bottomed holes are formed as therecessed portions, the direction of formation is defined as direction ofa line connecting every closest bottomed holes.

[0016] If the above-descried filled electrode lines are formed on themain surface of the semiconductor single crystal substrate having asurface orientation of {100} (may occasionally referred to simply as{100} substrate, hereinafter) along the <110> direction on the mainsurface of the substrate, the substrate may readily cleave along thedirection of formation thereof and result in fracture, similarly to thecase for the solar cell according to the first aspect. The mechanicalstrength of the substrate and consequently the resultant solar cell can,however, be improved to a large extent similarly to the solar cellaccording to the first aspect, if the direction of formation of thefilled electrode lines formed on the main surface of the {100} substrateis set in disagreement with the <110> direction. Moreover, in the methodof fabricating the solar cell, it is also possible to effectivelyprevent or suppress nonconformities such as factures of thesemiconductor single crystal substrate from occurring during thefabrication process of the solar cell, if the filled electrode lines areformed so that the direction of formation thereof is set in disagreementwith the <110> direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a drawing showing a relation between the groovedirection and crystallographic orientation of the substrate of an OECOsolar cell as an exemplary solar cell according to a first aspect ofthis invention;

[0018]FIG. 2 is a drawing exemplifying a sectional structure of anessential portion of the surface of the OECO solar cell as an exemplarysolar cell according to the first aspect of this invention;

[0019]FIG. 3A is a drawing showing a first example of a sectionalstructure of the surface grooves of the OECO solar cell as an exemplarysolar cell according to the first aspect of this invention;

[0020]FIG. 3B is a drawing showing a second example of the same;

[0021]FIG. 3C is a drawing showing a third example of the same;

[0022]FIG. 3D is a drawing showing a fourth example of the same;

[0023]FIG. 3E is a drawing showing a fifth example of the same;

[0024]FIG. 4 is a drawing showing an outline of process steps forfabricating the OECO solar cell as an exemplary solar cell according tothe first aspect of this invention;

[0025]FIG. 5A is a perspective view schematically showing a high speedrotary blade used for fabrication of the OECO solar cell as an exemplarysolar cell according to the first aspect of this invention;

[0026]FIG. 5B is a drawing of a first example of an edge profile of aperipheral blade provided to the high-speed rotary blade shown in FIG.5A;

[0027]FIG. 5C is a drawing showing a second example of the same;

[0028]FIG. 5D is a drawing showing a third example of the same;

[0029]FIG. 6A is a drawing showing a relation between the groovearrangement and crystallographic orientation of the substrate of anembodiment of a solar cell adopting grooves for electrode contactaccording to a second aspect of this invention;

[0030]FIG. 6B is a drawing showing a relation between arrangement ofbottomed holes and crystallographic orientation of the substrate of anembodiment of the solar cell adopting the bottomed holes according tothe second aspect of this invention;

[0031]FIG. 7 is a drawing exemplifying a sectional structure of anessential portion of the back surface of the solar cell according to thesecond aspect of this invention;

[0032]FIG. 8 is a drawing showing an outline of process steps forforming electrodes of the solar cell according to the second aspect ofthis invention;

[0033]FIG. 9 is a drawing showing a relation between arrangement ofholes for electrode contact and crystallographic orientation of thesubstrate of the solar cell according to the second aspect of thisinvention;

[0034]FIG. 10(a) is a drawing showing a positional relation betweenband-formed current collecting electrodes and bottomed holes used forthe solar cell according to the second aspect of this invention;

[0035]FIG. 10(b) is a view schematically showing a sectional structureof FIG. 10(a);

[0036]FIG. 11 is a perspective view of an essential portion of an OECOsolar cell of double-face-receiving type;

[0037]FIG. 12 is a drawing showing a setting of a test strip anddefinition of deflection in a deflection measurement test adopted in anexperiment of an Example of this invention;

[0038]FIG. 13 is a graph showing a groove direction dependence ofdeflection of the substrate in Example 1;

[0039]FIG. 14 is a graph showing a substrate thickness dependence ofdeflection of the substrate in Example 1, together with results ofComparative Example;

[0040]FIG. 15 is a graph showing a substrate thickness dependence ofdeflection of the substrate in Example 2, together with results ofComparative Example;

[0041]FIG. 16 is a graph showing a groove direction dependence ofdeflection of the substrate in Example 3; and

[0042]FIG. 17 is a graph showing an a dependence of deflection of thesubstrate in Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

[0043] The following paragraphs will describe best modes for carryingout this invention making reference to the attached drawings, where itis to be understood that this invention is by no means limited to thesebest modes.

FIRST EMBODIMENT 1

[0044]FIG. 1 is a drawing showing an embodiment of the solar cellaccording to the first aspect of this invention. FIG. 2 is a schematicenlarged sectional view showing a structure on the first main surface 24a side of the solar cell 1. The solar cell 1 is configured so that alarge number of grooves 2 of several-hundred-micrometers wide andapprox. 100 μm deep are formed in parallel on the first main surface 24a of a p-type silicon single crystal substrate sliced out from a siliconsingle crystal ingot. These grooves 2 can be carved en bloc using a setof hundreds to thousands of concentrically-joined rotary blades whichrotate all together, where it is also allowable to divide the carvingoperation into several numbers of run.

[0045] On the first main surface 24 a of the substrate having thegrooves 2 thus formed thereon, an emitter layer 4 is formed by thermallydiffusing phosphorus as an n-type dopant, so as to produce a p-njunction portion. Over the p-n junction, a thin silicon oxide film 5which functions as a tunnel insulating film is formed typically by thethermal oxidation process.

[0046] On the silicon oxide film 5, electrodes 6 are formed. Theelectrodes 6 are such as those formed by depositing an electrodematerial (e.g., metal such as aluminum) on the inner side face of thegrooves using a vacuum evaporation apparatus, where in the process ofdeposition, the substrate 1 is disposed as being relatively inclined ata least-necessary angle or more, so as to allow the electrode materialto deposit on the inner side face predominantly on one side in thewidth-wise direction of each groove, as described later. This is wherethe name of OECO comes from. While excessive electrode material depositsalso on the top surface of the projected ridges 23 formed between everyclosest grooves 2, 2 during the deposition, the excessive portion can beremoved using an etching solution such as hydrochloric acid solution.The entire portion of the first main surface 24 a of the substrate 1including the electrodes 6 are covered with a silicon nitride film 7which functions as a protective film and an anti-reflection film.

[0047] The individual grooves 2 of the solar cell 1 are formed in thedirection in disagreement with the <110> direction on the first mainsurface 24 a. This successfully raises the mechanical strength of thesolar cell 1. It is to be understood in this specification that anysubstrate is assumed as having a surface orientation of {100} if thecrystallographic principal axis of the single crystal substrate usedherein is an off-angled substrate having an angle of inclination of upto 6° away from the <100> direction.

[0048] As shown in FIG. 1, the main surface of the {100} substrate hastwo <110> directions normal to each other, and the grooves 2 are formedso as to agree with neither of the <110> directions. The direction offormation of each groove herein preferably crosses the <110> directionnearest to the direction of formation at an angle of 4° to 45° on theacute angle side. An angle smaller than 4° may result in only a limitedeffect of improvement in the mechanical strength of the solar cell ascompared with the case where the groove direction agrees with the <110>direction. On the other hand, it is geometrically impossible for theangle to exceed 45° away from both <110> directions. The direction offormation of the grooves becomes most distant from the <110> direction,which is a direction of easy cleavage, when it is in parallel with the<100> direction on the first main surface 24 a (i.e., the above angleequals to 45°), where maximum effect of raising the mechanical strengthof the solar cell is attainable.

[0049] Each groove 2 preferably has either rectangular contour in asectional view normal to the longitudinal direction thereof as shown inFIG. 3A, semicircular contour as shown in FIG. 3B, and V-contour asshown in FIG. 3C, because these morphologies can readily be obtained bycutting using a peripheral blade cutting. In particular for the purposeof reducing the series resistance of the solar cell, it is preferable toadopt the groove having the rectangular section as shown in FIG. 3A.

[0050] For the case where the groove 2 has a rectangular contour in asectional view normal to the longitudinal direction thereof as shown inFIG. 3A, or V-contour as shown in FIG. 3C, the contour has two edgeportions 2 a, 2 b abutted with each other as shown in FIG. 3D or FIG.3E. In an rectangular groove, the edge portions 2 a and 2 b correspondto the side wall and bottom of the groove, respectively, as shown inFIG. 3D, showing an angle of abutment of the both of 90° or around. Onthe other hand, V-formed groove has the edge portions 2 a, 2 b abuttedat an acute angle at the bottom of the groove. Both cases tend to resultin stress concentration and in lowered strength of the solar cell if theabutment is made at an acute angle. Provision of rounding R1 and R2 atthe abutment position of the edge portions 2 a, 2 b as viewed in asectional contour is now successful in further raising the mechanicalstrength of the solar cell.

[0051] Degrees of the rounding R1 and R2 are preferably set within arange which ensures a sufficient effect of preventing stressconcentration, and which does not ruin effects such as reduction in theseries resistance due to groove morphology, where a preferable range istypically 2 to 20 μm or around. The rounding can readily be produced bychemical etching after the grooves are carved by the peripheral bladecutting or the like. The chemical etching may be carried out in commonwith the etching for removing damage generated during the groovecarving. Amount of etching is preferably within a range from 5 to 20 μmor around so as to make the rounding fall within the above describedpreferable range. Aqueous potassium hydroxide solution is typically usedas a chemical etching solution.

[0052] Next paragraphs will describe an exemplary method of fabricatingthe solar cell 1.

[0053] First a p-type silicon single crystal ingot, which is prepared byadding a Group IIII element such as gallium to a high-purity silicon, isobtained, and p-type silicon single crystal substrates having a surfaceorientation of {100} are sliced out from the ingot. The p-type siliconsingle crystal substrate typically has a specific resistance of 0.5 to 5Ω·cm. Next, as shown in process (a) of FIG. 4, a plurality of parallelgrooves of 20 to 100 μm deep are formed on the first main surface 24 aof the p-type {100} substrate using a high-speed rotary blade in adirection in disagreement with <100>, typically in the <100> direction.While the silicon single crystal ingot may be prepared by either of theCZ (Czochralski) method and the FZ (floating zone melting) method, it ismore preferably prepared by the CZ method in view of the mechanicalstrength of the resultant substrate. While the thickness of thesubstrate of as small as 40 μm may ensure a sufficient mechanicalstrength, the thickness is more preferably 150 μm or more, and stillmore preferably 200 μm or more for the convenience of slicing operation.The effect of raising the mechanical strength through adoption of thegroove morphology specific to this invention becomes distinctive when asubstrate as thin as 230 μm or below is adopted.

[0054]FIG. 5A shows a schematic view of a high-speed rotary blade 107.The high-speed rotary blade 107 comprises a cylindrical portion (e.g.,103 mm in diameter, 165 mm in length) and a plurality of (e.g., 100 to200) peripheral cutting edges 108 for groove carving attached thereto.Profile of the cutting edge may properly be selected depending on thedesired morphology of the groove (see FIGS. 3A to 3E) from an edgehaving a rectangular section as shown in FIG. 5B, an edge having asemicircular section as shown in FIG. 5C, and an edge having an angularsection as shown in FIG. 5D. Heights 10, 10′, 10″ of the edges fallwithin a range from 50 to 100 μm for example, and widths 11, 11′, 11″ ofthe edges and distances 12, 12′, 12″ of edges are typically severalhundred micrometers or around for example, respectively. Availableblades include diamond blades (such as having the surface thereofuniformly adhered with diamond abrasive grain having a grain size of 5μm to 10 μm). The grooves 2 can be curved by cutting the main surface ofthe substrate 1 using the aforementioned high-speed rotary blade at acutting speed of approx. 1 to 4 cm per second while injecting a cuttingfluid. It is also allowable to use a dicer or a wire saw in place of thehigh-speed rotary blade.

[0055] Next, damage generated in the substrate after the grooveformation is removed by the aforementioned chemical etching. When thegroove has a rectangular form or V forms as shown in FIG. 3A or 3C,conditions for the etching for removing the damage are preferablyadjusted so as to properly round the grooves as shown in FIG. 3D or FIG.3E. After completion of the etching for removing damages, a texturestructure is then formed on the main surface of the substrate, which iscarried out as a surface roughening for reducing the reflection loss, byany publicly-known methods such as anisotropic etching. After thetexture is formed, the substrate is cleaned in an acidic aqueoussolution such as containing hydrochloric acid, sulfuric acid, nitricacid, hydrofluoric acid, or mixed solution thereof, where cleaning inhydrochloric acid is preferable from the viewpoints of economy andproduction efficiency.

[0056] Next, as shown in process (b) in FIG. 4, an emitter layer 4 isformed in the surficial portion of the substrate after cleaning. Methodsof forming the emitter layer may be any of the coating diffusion methodusing diphosphorus pentaoxide, ion implantation method for directlyimplanting phosphorus ion, and so forth, but a preferable method fromthe economical viewpoint is the vapor-phase diffusion method usingphosphoryl chloride. In an exemplary process, the n-type emitter layer 4can be formed by annealing the substrate in a phosphoryl chlorideatmosphere at 850° C. or around. Thickness of the emitter layer 4 istypically 0.5 μm or around, and the sheet resistance is within a rangefrom 40 to 100 Ω/□ or around. A phosphorus glass formed in the surficialportion of the substrate during the process is removed in a hydrofluoricacid solution.

[0057] Next, electrode is formed on a second main surface 24 b side.First, as shown in process (c) in FIG. 4, a silicon nitride layer 8 isformed as a passivation film on the second main surface 24 b. Thesilicon nitride layer 8 can be formed by the CVD (Chemical VaporDeposition) process. Any of the normal-pressure thermal CVD process,reduced-pressure thermal CVD process, photo CVD process and so forth isapplicable herein, where the remote plasma CVD process is particularlypreferable for this invention, because the process can proceed at lowertemperatures ranging from 350 to 400° C. or around, and can reduce thesurface recombination speed of the silicon nitride layer 8 to beobtained. It is to be noted that the direct thermal nitridation methodis not preferable because the process cannot afford a sufficientthickness of the resultant layer.

[0058] Next as shown in process (d) in FIG. 4, grooves 8 a for electrodeconnection are formed in thus-formed silicon nitride layer 8 so as toreach the underlying p-type silicon single crystal substrate 24 usingthe high-speed rotary blade similar to that shown in FIG. 5A. Profile ofthe cutting edge is selected from the rectangular form as shown in FIG.5B, semicircular form as shown in FIG. 5C, and angular form as shown inFIG. 5D depending on a desired sectional form of the grooves. After thegrooves 8 a are thus formed, an electrode 9 is then formed so as tocover the grooves 8 a together with the peripheral silicon nitride layer8 as shown in process (e) in FIG. 4. Although silver or copper isavailable as the electrode material herein, aluminum (including alloysthereof) is most preferable in view of economy and workability. Aluminumcan be deposited by either method of sputtering and vacuum evaporation.All processes for forming the electrode on the second main surface 24 bside thus complete.

[0059] Next as shown in process (f) in FIG. 4, the silicon oxide film 5is formed on the first main surface 24 a by the thermal oxidationmethod. The silicon oxide film 5 serves as a tunnel insulating filmbetween the electrode 6 on the first main surface 24 a and the substrate24, and preferably has a thickness of 5˜30 Å in order to optimizetunneling effect while preventing short-circuiting. The silicon oxidefilm 5 can be formed by any known methods including dry oxidation, wetoxidation, steam oxidation, pyrogenic oxidation, hydrochloric acidoxidation and so forth, and among others, dry oxidation is preferablyadopted since the method can ensure a high film quality and easy controlof the thickness.

[0060] On the substrate 24 having the silicon oxide film 5 alreadyformed thereon, the electrode 6 is formed on the inner side face(electrode-forming area) of the groove 2 on one side as viewed in thewidth-wise direction of the groove 2, typically to as thick as approx. 5μm by the oblique-angled vacuum evaporation process. Although aluminum(including alloys thereof) is most preferably used herein for theelectrode material, the material is not limited thereto, and other metalsuch as silver, copper or the like can be used. More specifically, thesubstrate 24 is placed in a vacuum evaporation apparatus so as toincline the principal axis thereof at 70° to 85° away from thereferential position, where the referential position is defined as aposition where the first main surface 24 a is oriented to theevaporation source so that the extending direction of the grooves 2crosses normal to the evaporation source. This placement successfullyallows the electrode material to deposit predominantly on the inner sideface of the grooves 2 on one side as viewed in the width-wise direction.The deposition is preferably effected only after the degree of vacuum inthe apparatus reaches to a level as low as 2×10⁻⁵ Pa or below, and thedeposition speed is adjusted to 10˜15 Å/sec (but not limited thereto).Next as shown in process (g) in FIG. 4, the substrate 24 having theelectrodes 6 deposited thereon is dipped into an acidic aqueous solutioncontaining hydrochloric acid, sulfuric aid, nitric acid, hydrofluoricacid or mixed solution thereof, to thereby remove the unnecessaryportion of the electrode material deposited on the top surfaces of theprojected ridges 23 generated between the adjacent grooves 2, 2. Theremoval is preferably proceeded typically in hydrochloric acid solutionbecause an appropriate etching rate is attainable, and becauseunnecessary compound formation due to reaction with the underlying layeris less likely to occur.

[0061] On the substrate 24 after completion of the above processes, abus bar electrode (not shown) is formed by a publicly-known method, andthe silicon nitride film 7, which serves as a passivation film and ananti-reflection film, is uniformly formed on the first main surface 24 ato a thickness of 600˜700 Å typically by the remote plasma CVD process(process (h) in FIG. 4), to thereby complete the final solar cell 1.

[0062] In the solar cell of this invention, it is also allowable to forma light-receiving-element structure of the OECO solar cell also on thesecond main surface 24 b side of the substrate 24 as shown in FIG. 11,similarly to as on the first main surface 24 a side. In this case, it ispreferable to adopt a structure in which a plurality of grooves 2 nearlyparallel with each other are formed on the second main surface 24 b ofthe silicon single crystal substrate (semiconductor single crystalsubstrate) 24 in directions in disagreement with the <110> direction onthe first main surface 24 a and so as to cross the directions offormation of the grooves 2 on the first main surface 24 a, and in whichan electrode for extracting output is disposed on the inner side face ofeach groove 2 on the second main surface 24 b on one side in thewidth-wise direction of the groove 2. The difference in the directionsof formation of the grooves 2 on the second main surface 24 b side andof the grooves 2 on the first main surface 24a side successfully resultsin improved mechanical strength of the solar cell having grooves formedon both main surfaces thereof. It is most preferable to form the grooves2 so that the direction of formation thereof on the second main surface24 b cross nearly normal to those on the first main surface 24 a in viewof optimizing the mechanical strength.

SECOND EMBODIMENT

[0063]FIG. 6A is a drawing showing an embodiment of a solar cell 201according to the second aspect of this invention. FIG. 7 is an enlargedschematic sectional view showing a structure on a first main surface 203a side of the solar cell 201. In the solar cell 201, a large number ofnearly-parallel groove portions 202 of approx.several-hundred-micrometers wide and approx. 100 μm deep are formed on afirst main surface 203 a (this surface is defined as the back surface inthis embodiment) of a p-type silicon single crystal substrate 203 (alsosimply referred to as substrate 203), and the groove portions 202 arefilled with an electric conductor 205 to thereby form filled electrodelines 240 (FIG. 6A). These groove portions 202 can be carved en blocusing a set of hundreds to thousands of concentrically-joined rotaryblades which rotate all together, where it is also allowable to dividethe carving operation into several numbers of run. While the descriptionof this embodiment will be made on a p-type silicon single crystalsubstrate 203 sliced out from a silicon single crystal ingot, it is tobe understood that this invention is by no means limited thereto.

[0064] In this embodiment, an insulating film 204 is formed on the firstmain surface 203 a of the p-type silicon single crystal substrate(semiconductor single crystal substrate) 203, and the filled electrodelines 240 are formed so that the electric conductor 205 filled thereinmakes contact with the p-type silicon single crystal substrate 203 in aform that the groove portions 202 composing the filled electrode lines240 penetrate the insulating film 204.

[0065] In the solar cell 201 of this embodiment, a current collectingelectrode communicating with the filled electrode lines 240 is formed onthe first main surface 203 a, and is preferably formed as a coverelectrode layer 210 covering entire surface of the first main surface203 a. The insulating film 204 formed on the p-type silicon singlecrystal substrate 203 may preferably comprise a silicon oxide film,silicon nitride film and the like.

[0066] In the above-described solar cell 201 of this embodiment, thefirst main surface 203 a of the p-type silicon single crystal substrate203 has a surface orientation of {100}, and the individual grooveportions 202 composing the filled electrode lines are formed in thedirection in disagreement with the <110> direction on the first mainsurface 203a. This successfully raises the mechanical strength of thesolar cell 201. It is to be understood in this specification that anysubstrate is assumed as having a surface orientation of {100} if thecrystallographic principal axis of the single crystal substrate usedherein is an off-angled substrate having an angle of inclination of upto 6° away from the <100> direction.

[0067] As shown in FIG. 6A, the first main surface 203 a of the {100}substrate has two <110> directions normal to each other, and thedirection of formation of the groove portions 202 is set as being indisagreement with both of the <110> directions. The direction offormation of the groove portions 202 and the direction nearest theretopreferably cross at an angle of 4° to 45° on the acute angle side. Anangle smaller than 40 may result in only a limited effect of improvementin the mechanical strength of the solar cell 201 as compared with thecase where the groove direction agrees with the <110> direction. On theother hand, it is geometrically impossible for the angle to exceed 45°away from both of the <110> directions. The direction of formation ofthe groove portions 202 becomes most distant from the <110> direction,which is a direction of easy cleavage, when it is in parallel with the<100> direction on the first main surface 203 a (i.e., the above angleequals to 45°), where maximum effect of raising the mechanical strengthof the solar cell 201 is attainable.

[0068] Next, a solar cell 201′ according to another embodiment of thisinvention is shown in FIG. 6B, FIG. 10(a) and FIG. 10(b). In the solarcell 201′, a great number of bottomed holes 214 typically having adiameter of several hundred micrometers and a depth of 5 to 50 μm oraround are formed on a main surface 203 a′ of a p-type silicon singlecrystal substrate 203′, in a form that every closest bottomed holes 214are linearly aligned at regular intervals. These bottomed holes 214 arefilled with an electrode conductor 205′ (see FIG. 7), and as aconsequence arrays in which every closest bottomed holes 214 arelinearly aligned at regular intervals configure filled electrode lines240′. The filled electrode lines 240′ are formed so as to penetrate theinsulating film 204′ as shown in FIG. 7. Assuming now that the directionof a line connecting every closest bottomed holes 214 as the directionof formation of the filled electrode lines 240′, such direction offormation of the filled electrode lines 240′ is in disagreement with the<110> direction on the first main surface 203′. The direction offormation of each filled electrode line 240 preferably crosses the <110>direction nearest thereto at an angle of 4° to 45° on the acute angleside as shown in FIG. 6B, similarly to the case where the filledelectrode lines 240 are formed by the aforementioned groove portions 202(see FIG. 10(a)), and more preferably lies in parallel with the <100>direction on the first main surface 203 a′ (at an angle of 45° to the<110> direction).

[0069] Next paragraphs will describe a method of fabricating the solarcells 201 (FIG. 6A) and 201′ (FIG. 6B) referring to FIG. 8. It is to beunderstood that this invention is by no means limited to the solar cellsfabricated by this method. Because the fabrication method of the solarcells 201 and 201′ are similar to a large extent, any descriptions forthe common portion will be represented by those for the solar cell 201,and corresponded portions of the solar cell 201′ will be given in theparentheses so as to avoid redundant description.

[0070] First a silicon single crystal ingot, which is prepared by dopinga Group III element such as boron or gallium to a high-purity silicon,is obtained, and p-type silicon single crystal substrates 203 (203′)having a surface orientation of {100} are sliced out from the ingot. Thep-type silicon single crystal substrate 203 (203′) typically has aspecific resistivity of 0.5 to 5 Ω·cm. While the silicon single crystalingot may be prepared by either of the CZ (Czochralski) method and theFZ (floating zone melting) method, it is more preferably prepared by theCZ method in view of the mechanical strength of the resultant substrate.The effect of raising the mechanical strength through adoption of thegroove morphology specific to this invention becomes distinctive when asubstrate as thin as 230 μm or below is adopted.

[0071] On the first main surface (having a surface orientation of {100})of the as-cut, p-type silicon single crystal substrate 203 (203′), atexture structure is then formed by any publicly-known method. After thetexture structure is formed, the substrate is cleaned in an acidicaqueous solution such as containing hydrochloric acid, sulfuric acid,nitric acid, hydrofluoric acid, or mixed solution thereof, wherecleaning in hydrochloric acid is preferable from the viewpoints ofeconomy and production efficiency. Purpose of the texture formationresides in surface roughening for reducing the reflection loss. Thep-type silicon single crystal substrate 203 (203′) after completion ofthe above processes is expressed by process (a) in FIG. 8.

[0072] On the first main surface (back surface) 203 a (203 a′) of thep-type silicon single crystal substrate 203 (203′), an insulating film204 (204′) such as a silicon oxide film or silicon nitride film isformed by any publicly-known method, which is typified by the CVD(Chemical Vapor Deposition) process, to a thickness of 50 to 500 nm(process (b) in FIG. 8). Any of the normal-pressure thermal CVD process,reduced-pressure thermal CVD process, photo CVD process and so forth isapplicable herein, where the remote plasma CVD process is particularlypreferable for this invention, because the process can proceed at lowertemperatures ranging from 350 to 400° C. or around, and can reduce thesurface recombination speed of the resultant insulating film 204 (204′)comprising silicon oxide or silicon nitride.

[0073] For the case where the second main surface (defined as the topsurface in this embodiment, although not shown) is used as thelight-receiving surface, it is also allowable in this stage to form theemitter layer (not shown) in the light-receiving surface by the vaporphase diffusion process using phosphoryl chloride, because theinsulating film is also effective as a mask for blocking phosphorusdiffusion. That is, diffusion of phosphorus into the first main surface203 a (203 a′) is blocked by the insulating film 204 (204′) formed onthe first main surface 203 a (203a′). Other possible methods for formingthe emitter layer include the coating diffusion method usingdiphosphorus pentaoxide, ion implantation method for directly implantingphosphorus ion, and so forth, where a preferable method from theeconomical viewpoint is the aforementioned vapor phase diffusionprocess. In an exemplary process, the n-type emitter layer can be formedby annealing the p-type silicon single crystal substrate in a phosphorylchloride atmosphere at 850° C. or around. Thickness of the resultantemitter layer is typically 0.5 μm or around, and the sheet resistancefalls within a range from 40 to 100 Ω/□ or around. A phosphorus glassformed in the surficial portion of the substrate during the process isremoved in a hydrofluoric acid solution.

[0074] The n-type emitter layer is thus formed in the surficial portionof the second main surface (top surface) which serves as thelight-receiving surface, and a p-n junction portion is thus formed inthe substrate.

[0075] Next paragraphs will describe a method of forming the filledelectrode lines 240 (240′) on the first main surface 203 a (203 a′) ofthe p-type silicon single crystal substrate 203 (203′). For the casewhere the filled electrode lines 240 are formed by first forming thegrooves 202, a plurality of nearly-parallel groove portions are formedon the main surface 203 a of the p-type silicon single crystal substrate203 using a high-speed rotary blade, and by filling the groove portions202 with the electric conductor 205 (processes (c) and (d) in FIG. 8).More specifically, the groove portions 202 for electrode contact areformed in the insulating layer 204. The groove portions 202 are carvedtypically in the <100> direction on the first main surface 203 a of thesubstrate 203, using the high-speed rotary blade 107 as shown in FIG.5A. Height and profile of the peripheral cutting edge 108 of thehigh-speed rotary blade 107 can properly be selected depending ondesired morphology of the groove portions 202 to be formed on the firstmain surface 203 a of the p-type silicon single crystal substrate 203.Height of the cutting edge is typically 50 to 100 μm, and width(corresponds with the width of the groove portions 202 to be formed) andpitch (corresponded with the pitch of the groove portions 202 to beformed) are several-hundred-micrometers or around, respectively. Usingsuch high-speed rotary blade 107, the substrate is carved at a cuttingspeed of approx. 1 to 4 cm per second while injecting a cutting water,to thereby form the groove portions 202. The height and so forth of theperipheral cutting edge 108 are finely adjusted so as to adjust thedepth of the groove portions 202 to approx. 5 to 50 μm. Because thethickness of the insulating film 204 formed on the first main surface203 a of the substrate 203 is approx. 50 to 500 nm, the groove portions202 formed in the thickness within the above range can penetrate theinsulating film 204. This successfully completes the filled electrodelines 240 in which the electric conductor 205 filled in the grooveportions 202 contacts with the p-type silicon single crystal substrate203.

[0076] On the other hand, as shown in FIG. 10A and FIG. 10B, for thecase where the solar cell 201′ has the filled electrode lines 240′composed of the bottomed holes 214 bored in the p-type silicon singlecrystal substrate 203′, the bottomed holes 214 are first linearly formedat regular intervals on the main surface 203 a′ side of the p-typesilicon single crystal substrate 203′ by irradiating laser thereon, sothat the line connecting every closest bottomed holes 214 is indisagreement with the <110> direction. The bottomed holes 214 are thenfilled with the electric conductor 205′ for composing the electrodes, tothereby complete the filled electrode lines 240′. Lasers available forforming the bottomed holes 214 include carbon dioxide gas laser, argonlaser, YAG laser, ruby laser and excimer laser. Among others, excimerlaser such as KrF laser and Nd:YAG laser can preferably be used becausethese lasers can ensure fine processing to as fine as wavelength of thelaser beam, and the processing can be proceeded in the air. Anymorphology of circular form and rectangular form are allowable for thebottomed holes 214. The bottomed holes 214 are linearly aligned whilekeeping regular intervals between every closest bottomed hole 214.Assuming now a set of thus-linearly-aligned bottomed holes 214 composesone filled electrode line 240′, a plurality of the filled electrodelines 240′ are periodically arranged on the first main surface 203 a′while keeping regular intervals therebetween. FIG. 9 is a schematicdrawing showing a relation between arrangement of the bottomed holes 214and directionality of the substrate. The direction 212 of a lineconnecting every closest bottomed holes 214 formed by laser irradiation(direction of formation of the filled electrode line 240′) is set in adirection in disagreement with the <110> direction on the main surfaceof the substrate 203′. It is preferable that also the direction 213 of aline connecting every second-closest bottomed hole, differing from thedirection 212, is in disagreement with the <110> direction.

[0077] Conditions for the laser irradiation for forming the bottomedholes 214 can properly be determined depending on types of the laser,thickness of the insulating film 204′, diameter of the bottomed holes214, and so forth. For the case where pulse oscillation is adopted, thefrequency is preferably within a range from 1 Hz to 100 kHz, and thelaser preferably has an average output of 10 mW to 1 kW. Because theinsulating film 204′ formed herein has a thickness of 50 to 500 nm, itis necessary to irradiate laser having an output energy large enough toremove the insulating film 204′ having at least the above-describedthickness.

[0078] The filled electrode lines 240 (240′) are thus formed by fillingthe groove portions 202 or bottomed holes 214 with the electricconductor 205 (205′), and the cover electrode layer 210 is formed in athickness of 0.5 to 2 μm over the entire portion of the first mainsurface 203 a (203 a′) (process (d) in FIG. 8). In this process, theelectric conductor 205 (205′) and cover electrode layer 210 aresuccessively formed in the same process step, as being started from thestatus expressed by process (c) in FIG. 8.

[0079] While the electric conductor 205 (205′) and cover electrode layer210 can be composed of metals such as silver, copper and the like, orconductive indium oxide, tin oxide and the like, most preferablematerial is aluminum in view of economy and workability. The electricconductor 205 (205′) and cover electrode layer 210 can be deposited byany known methods of sputtering, vacuum evaporation, screen printing andso forth. Moreover, the cover electrode layer 210 can, of course, bedeposited by uniformly over the entire portion of the first main surface203 (203′) as described in the above, it is also allowable to form alinear or band-formed current collecting electrode 217 (also referred toas band electrode 217, hereinafter) on the filled electrode line 240′which is formed by filling the grooves (not shown) or bottomed holes 14with the electric conductor 5′ as shown in FIGS. 10A and 10B typicallyusing a mask for electrode formation. It is also allowable to form thecurrent collecting electrode 217 having a linear form or band form in adirection at 4 to 90° away from the direction of formation of the filledelectrode line 240′. This is successful in further raising themechanical strength of the semiconductor single crystal substrate(p-type silicon single crystal substrate), and consequently of the solarcell. Although FIG. 10(a) and 10(b) showed the case where the filledelectrode lines 240′ were composed of the bottomed holes 214, thesimilar current collecting electrode 217 as described in the above canbe formed also for the case where the filled electrode lines 240 arecomposed of the grooves 202.

[0080] After the electric conductor 205 for composing the electrodes,cover electrode layer 210 and the band-formed electrode 217 are formedon the first main surface 203 a as described in the above, theanti-reflection film and electrodes are formed on the second mainsurface according to the publicly-known methods. Materials for composingthe anti-reflection film include silicon oxide, silicon nitride, ceriumoxide, alumina, tin dioxide, titanium dioxide, magnesium fluoride,tantalum oxide, and a double-layered film composed of any two of thesematerials, where all materials are available without any problems. Theanti-reflection film can be formed by the PVD process, CVD process orthe like, where any process is successful. In view of obtaining ahigh-conversion-efficiency solar cell, it is preferable to compose theanti-reflection film with a silicon nitride film formed by theremote-plasma CVD process, because thus-formed film has a small surfacerecombination speed. The electrodes on the second main surface (topsurface) are formed by vacuum evaporation, plating, printing or thelike. Although any of these methods are available, printing ispreferably used in pursuit of a low cost and high throughput. Typicalscreen printing uses a silver paste composed of silver powder, glassfrit and organic binder mixed with each other, and the electrodes can beformed by annealing the printed paste.

[0081] Which processes for the top surface (second main surface) andback surface (first main surface) should preceed is of no problem.Although the aforementioned embodiment described the case where thefilled electrode lines 240 (240′) are formed on the first main surface203 a (203 a′) of the p-type silicon single crystal substrate 203(203′), and the second main surface is used as the light-receivingsurface, this invention is by no means limited thereto, and similareffects will be shown by the solar cell in which the electrodes arecomposed of the filled electrode lines obtained by forming the groovesor bottomed holes on the second main surface which serves as thelight-receiving surface.

[0082] To confirm operation and effects of the solar cells according tothe first aspect of this invention, the following experiments werecarried out.

EXAMPLE 1

[0083] On the first main surfaces of the boron-doped {100} p-typesilicon single crystal substrates (specific resistance=1 Ω·cm)respectively having either thickness of 250, 200 and 150 μm, parallelgrooves having a rectangular section were formed respectively in eitherdirection at angles of 0°, 30°, 45°, 60° and 90° to the <110> direction,using the high-speed rotary blade shown in FIG. 5A. Width, depth andpitch of the grooves were determined as 450 μm, 50 μm and 600 μm,respectively. The solar cells were fabricated according to the processsteps previously described referring to FIG. 4. Energy conversionefficiency of the solar cells measured under the standard conditionswere found to range from 18 to 20%. Each solar cell was cut into a teststrip of 18 mm wide and 100 mm long using a dicer. The test strip wasthen set on a three-point bending tester as shown in FIG. 12 so that thetest strip 13′ was supported at both ends thereof by two round-rodsupport members 14, 14′ (outer diameter of support portion=28 mm, spanbetween the support portions=80 mm) while directing the groove-formedsurface (first main surface) facedown, and also aligning the groovedirection with the axial directions of the round-rod support members 14,14′. Three point bending test was then carried out by contacting anotherround-rod support member 15′ of same dimensions to the center of thelongitudinal direction of a portion of the test strip 13′ which fallsbetween the round-rod support members 14, 14′, and by applying adownward load for bending through the round-rod support member 15′.Maximum displacement 16 of the test strip 13 immediately before fracturewas determined from a displacement-load curve of the round-rod supportmember 15′, and this was defined as “deflection”. The individual sampleswere measured in a similar manner.

[0084] For comparison, solar cells were fabricated using the siliconsingle crystal substrates having the individual thickness withoutforming the grooves, but by similarly carrying out etching for damageremoval, texture formation, phosphorus diffusion, electrode formationusing aluminum on the second main surface, and deposition of the siliconnitride film on the first main surface. The solar cells were thensimilarly subjected to the deflection measurement. The test piecesherein were cut out from the substrate so that the longitudinaldirection thereof was in agreement with the <100> direction of thesubstrate. Therefore, the axial direction of the round-rod supportmembers 14, 14′ herein becomes in parallel with the <100> directionnormal to another <100> direction that lies in parallel with thelongitudinal direction of the test piece.

[0085]FIG. 13 shows a groove direction dependence of deflection of the150-μm thick substrate. The curve indicates that the deflection becomesmaximum when the groove direction is at 45° away from the <110>direction, that is, when the grooves are formed in the <100> direction,showing an excellent mechanical strength. FIG. 14 shows substratethickness dependences of deflection in comparison with ComparativeExample (no grooves). The results indicate that the deflection increasesas the thickness of the substrate decreases, and effect of improving themechanical strength through adjustment of the groove direction becomesmore distinctive as the OECO solar cells are thinned. It is also foundthat the groove formation results in a larger deflection, and that thegrooved substrate is more excellent in the mechanical strength.

EXAMPLE 2

[0086] On the first main surfaces of the {100} p-type silicon singlecrystal substrates respectively having either thickness of 250, 200 and150 μm, parallel grooves having a rectangular section were formedrespectively in either direction at angles of 0° and 45° to the <110>direction by a method similar to as described in Example 1, and on thesecond main surfaces thereof, a plurality of grooves having arectangular section were formed normal to the direction of formation ofthe grooves on the first main surface. Width, depth and pitch of thegrooves were determined as 450 μm, 50 μm and 600 μm, respectively. Thelight-receiving element structures were formed on both main surfacesaccording to the method explained with reference to FIG. 4, to therebyfabricate a double-face-receiving-type OECO solar cell.

[0087] A schematic view of thus fabricated solar cell was shown in FIG.11. Test pieces were cut out from these solar cells similarly to asdescribed in Example 1, and subjected to the deflection measurement.Each test pieces was prepared so that the longitudinal direction thereofwas in agreement with the groove direction on the first main surfaceside, and, during the measurement, was disposed so that the groovedirection on the first main surface side was in parallel with theround-rod support members 14, 14′. FIG. 15 shows substrate thicknessdependences of the deflection in comparison with Comparative Example. Itwas found that the deflection became maximum when the grooves wereformed in the <100> direction and increased as the thickness of thesubstrate decreased, and an effect of improving the mechanical strengthbecame more distinctive as the OECO solar cells were thinned.

[0088] As is obvious from the experimental results in the above, in thefabrication of the OECO solar cells, shifting of the groove directionfrom the <110> direction successfully raised the crack resistance andincreased the mechanical strength of the solar cells (FIG. 13). Inparticular, the groove direction aligned with the <100> directionmaximized the mechanical strength. This effect became more distinctiveas the thickness of the substrate decreased (FIG. 14), and this provedan advantage in cost reduction of the solar cells. It was also madeclear that the solar cell, having the grooves also on the second mainsurface which lay normal to those on the first main surface (FIG. 11),was also successful in retaining a mechanical strength almost equivalentto that shown by the solar cell having no groove (FIG. 15), and thisproved that this invention is also advantageous in fabricating thedouble-face-receiving-type OECO solar cells.

[0089] To confirm operation and effects of the solar cells according tothe second aspect of this invention, the following experiments werecarried out.

EXAMPLE 3

[0090] On the first main surfaces (back surface) of the boron-doped{100} p-type silicon substrates (specific resistance=1 Ω·cm) having athickness of 150 μm, a silicon nitride film of 100 nm thick was formed,and parallel grooves were formed respectively in either direction atangles of 0°, 30°, 45°, 60° and 90° to the <110> direction, using adicer (Model DAD-2H/6H, product of Disco Corporation). Width, depth andpitch of the grooves were determined as 450 μm, 50 μm and 600 μm,respectively. Aluminum was then deposited over the entire portion of thefirst main surface to thereby form a back electrode. On the second mainsurface (top surface, or light-receiving surface), the emitter layer,anti-reflection film, finger electrode and bus bar electrode weresequentially formed by the publicly-known methods, to thereby completesingle-face-receiving-type solar cells. Conversion efficiencies of thesesolar cells were found to range from 15 to 17%.

[0091] These solar cells were cut using the dicer to produce 18×100 mm²test strips, and the test strips were subjected to the three-pointbending test so as to measure deflection according to the systemillustrated in FIG. 12.

[0092]FIG. 16 shows groove direction dependence of the deflection. Theresults indicate that the deflection becomes maximum when the groovedirection is at 45° away from the <110> direction, that is, when thegrooves are formed in the <100> direction, showing an excellentmechanical strength.

EXAMPLE 4

[0093] On the back surfaces of the boron-doped {100} p-type siliconsubstrates (specific resistance=1 Ω·cm) having a thickness of 150 μm,which is similar to those used in Example 3, a silicon nitride film of100 nm thick was formed, and a plurality of bottomed holes were formedusing KrF excimer laser so that every closest bottomed holes arelinearly aligned at regular intervals. Distance between the adjacentbottomed holes and diameter of the opening portion were set to 600 μmand 450 μm, respectively, and depth of the bottomed holes was set toapprox. 50 μm by adjusting laser output (e.g., laser energy density=23.6J/cm², oscillation frequency=100 Hz, continuous irradiation time=approx.2.3 seconds). Assuming now that the angle between the direction of aline connecting every closest bottomed holes and the <110> direction asα°, and parallel grooves were formed respectively in either direction atangles α° of 0°, 30°, 45°, 60° and 90°. Aluminum was then deposited overthe entire portion of the first main surface to thereby form a backelectrode. On the second main surface (top surface, or light-receivingsurface), the emitter layer, anti-reflection film, finger electrode andbus bar electrode were sequentially formed by the publicly-knownmethods, to thereby complete single-face-receiving-type solar cells.Conversion efficiencies of these solar cells were found to range from 14to 17%.

[0094] These solar cells were cut using the dicer to produce 18×100 mm²test strips, and the test strips were subjected to the three-pointbending test similarly to as described in Example 3.

[0095]FIG. 17 shows α dependence of the deflection. The results indicatethat the deflection becomes maximum when the groove direction is ataround 30° and 60° away from the <110> direction, that is, when the lineconnecting every closest bottomed hole is in disagreement with the <110>direction, showing an excellent mechanical strength.

1. A solar cell configured so as to have a plurality of grooves nearlyparallel with each other formed on a first main surface of asemiconductor single crystal substrate having a surface orientation ofnearly {100}, each of the grooves having an electrode for extractingoutput disposed on the inner side face thereof on one side in thewidth-wise direction thereof, and the grooves being formed on the firstmain surface in directions in disagreement with the <110> direction. 2.The solar cell as claimed in claim 1, wherein the direction of formationof each groove crosses the <110> direction nearest to the direction offormation at an angle of 4° to 45° on the acute angle side.
 3. The solarcell as claimed in claim 2, wherein each of the grooves is formed in adirection parallel to the <100> direction on the first main surface. 4.The solar cell as claimed in any one of claims 1 to 3, wherein a contourof each of the grooves in a sectional view normal to the longitudinaldirection thereof has any one of rectangular, V- and semicircular forms.5. The solar cell as claimed in any one of claims 1 to 4, wherein acontour of each of the grooves in a sectional view normal to thelongitudinal direction thereof has two edge portions abutted with eachother, and the abutted portion of the edge portions is rounded.
 6. Thesolar cell as claimed in any one of claims 1 to 5, configured so that aplurality of grooves nearly parallel with each other are formed also ona second main surface of the semiconductor single crystal substrate indirections in disagreement with the <110> direction on the first mainsurface and so as to cross the directions of formation of the grooves onthe first main surface, and so that an electrode for extracting outputis disposed on the inner side face of each groove on the second mainsurface on one side in the width-wise direction of the groove.
 7. Thesolar cell as claimed in claim 6, wherein the directions of formation ofthe grooves on the second main surface cross nearly normal to thedirections of formation of the grooves on the first main surface.
 8. Asolar cell configured so as to have a plurality of filled electrodelines, having an electric conductor for composing electrodes forextracting output filled therein, on at least either main surface sideof a semiconductor single crystal substrate having a surface orientationof nearly {100}, the filled electrode lines being formed in directionsin disagreement with the <110> direction on the main surface.
 9. Thesolar cell as claimed in claim 8, wherein the direction of formation ofeach filled electrode line crosses the <110> direction nearest to thedirection of formation at an angle of 4° to 45° on the acute angle side.10. The solar cell as claimed in claim 8 or 9, wherein each of thegrooves is formed in a direction parallel to the <100> direction on themain surface.
 11. The solar cell as claimed in any one of claims 8 to10, wherein the filled electrode lines are formed on the first mainsurface of the semiconductor single crystal substrate, and the secondmain surface serves as a light-receiving surface.
 12. The solar cell asclaimed in claim 11, wherein an insulating film is formed on the firstmain surface of the semiconductor single crystal substrate, and thefilled electrode lines are formed so that the electric conductor filledtherein makes contact with the semiconductor single crystal substrate ina form that the filled electrode lines penetrate the insulating film.13. The solar cell as claimed in claim 12, wherein a current collectingelectrode communicating with the filled electrode lines formed on thefirst main surface is formed again on the first main surface.
 14. Thesolar cell as claimed in claim 13, wherein the current collectingelectrode is a cover electrode layer covering entire portion of thefirst main surface.
 15. The solar cell as claimed in claim 13, whereinthe current collecting electrode is formed so as to have a band patternor line pattern on the filled electrode lines.
 16. The solar cell asclaimed in claim 13, wherein the current collecting electrode is formedso as to have a band pattern or line pattern, and in the directioninclined at 4° to 90° away from the direction of formation of the filledelectrode lines.
 17. A method of fabricating a solar cell comprising astep of forming a plurality of filled electrode lines, having anelectric conductor for composing electrodes for extracting output filledtherein, on at least either main surface side of a semiconductor singlecrystal substrate having a surface orientation of nearly {100}, indirections in disagreement with the <110> direction on the main surface.18. The method of fabricating a solar cell as claimed in claim 17,wherein the filled electrode lines are formed by first forming aplurality of groove portions nearly parallel with each other on the mainsurface of the semiconductor single crystal substrate, and then byfilling the groove portions with the electric conductor for composingelectrodes.
 19. The method of fabricating a solar cell as claimed inclaim 17, wherein the filled electrode lines are formed by first forminga plurality of bottomed holes on the main surface of the semiconductorsingle crystal substrate by laser irradiation, the bottomed holes beinglinearly aligned at regular intervals, and the direction of a lineconnecting every closest bottomed holes being in disagreement with the<110> direction, and then by filling the groove portions with theelectric conductor for composing electrodes.